//===- Reassociate.cpp - Reassociate binary expressions -------------------===// // // The LLVM Compiler Infrastructure // // This file was developed by the LLVM research group and is distributed under // the University of Illinois Open Source License. See LICENSE.TXT for details. // //===----------------------------------------------------------------------===// // // This pass reassociates commutative expressions in an order that is designed // to promote better constant propagation, GCSE, LICM, PRE... // // For example: 4 + (x + 5) -> x + (4 + 5) // // Note that this pass works best if left shifts have been promoted to explicit // multiplies before this pass executes. // // In the implementation of this algorithm, constants are assigned rank = 0, // function arguments are rank = 1, and other values are assigned ranks // corresponding to the reverse post order traversal of current function // (starting at 2), which effectively gives values in deep loops higher rank // than values not in loops. // //===----------------------------------------------------------------------===// #include "llvm/Transforms/Scalar.h" #include "llvm/Function.h" #include "llvm/Instructions.h" #include "llvm/Type.h" #include "llvm/Pass.h" #include "llvm/Constant.h" #include "llvm/Support/CFG.h" #include "llvm/Support/Debug.h" #include "llvm/ADT/PostOrderIterator.h" #include "llvm/ADT/Statistic.h" using namespace llvm; namespace { Statistic<> NumLinear ("reassociate","Number of insts linearized"); Statistic<> NumChanged("reassociate","Number of insts reassociated"); Statistic<> NumSwapped("reassociate","Number of insts with operands swapped"); class Reassociate : public FunctionPass { std::map<BasicBlock*, unsigned> RankMap; std::map<Value*, unsigned> ValueRankMap; public: bool runOnFunction(Function &F); virtual void getAnalysisUsage(AnalysisUsage &AU) const { AU.setPreservesCFG(); } private: void BuildRankMap(Function &F); unsigned getRank(Value *V); bool ReassociateExpr(BinaryOperator *I); bool ReassociateBB(BasicBlock *BB); }; RegisterOpt<Reassociate> X("reassociate", "Reassociate expressions"); } // Public interface to the Reassociate pass FunctionPass *llvm::createReassociatePass() { return new Reassociate(); } void Reassociate::BuildRankMap(Function &F) { unsigned i = 2; // Assign distinct ranks to function arguments for (Function::arg_iterator I = F.arg_begin(), E = F.arg_end(); I != E; ++I) ValueRankMap[I] = ++i; ReversePostOrderTraversal<Function*> RPOT(&F); for (ReversePostOrderTraversal<Function*>::rpo_iterator I = RPOT.begin(), E = RPOT.end(); I != E; ++I) RankMap[*I] = ++i << 16; } unsigned Reassociate::getRank(Value *V) { if (isa<Argument>(V)) return ValueRankMap[V]; // Function argument... if (Instruction *I = dyn_cast<Instruction>(V)) { // If this is an expression, return the 1+MAX(rank(LHS), rank(RHS)) so that // we can reassociate expressions for code motion! Since we do not recurse // for PHI nodes, we cannot have infinite recursion here, because there // cannot be loops in the value graph that do not go through PHI nodes. // if (I->getOpcode() == Instruction::PHI || I->getOpcode() == Instruction::Alloca || I->getOpcode() == Instruction::Malloc || isa<TerminatorInst>(I) || I->mayWriteToMemory()) // Cannot move inst if it writes to memory! return RankMap[I->getParent()]; unsigned &CachedRank = ValueRankMap[I]; if (CachedRank) return CachedRank; // Rank already known? // If not, compute it! unsigned Rank = 0, MaxRank = RankMap[I->getParent()]; for (unsigned i = 0, e = I->getNumOperands(); i != e && Rank != MaxRank; ++i) Rank = std::max(Rank, getRank(I->getOperand(i))); DEBUG(std::cerr << "Calculated Rank[" << V->getName() << "] = " << Rank+1 << "\n"); return CachedRank = Rank+1; } // Otherwise it's a global or constant, rank 0. return 0; } bool Reassociate::ReassociateExpr(BinaryOperator *I) { Value *LHS = I->getOperand(0); Value *RHS = I->getOperand(1); unsigned LHSRank = getRank(LHS); unsigned RHSRank = getRank(RHS); bool Changed = false; // Make sure the LHS of the operand always has the greater rank... if (LHSRank < RHSRank) { bool Success = !I->swapOperands(); assert(Success && "swapOperands failed"); std::swap(LHS, RHS); std::swap(LHSRank, RHSRank); Changed = true; ++NumSwapped; DEBUG(std::cerr << "Transposed: " << *I /* << " Result BB: " << I->getParent()*/); } // If the LHS is the same operator as the current one is, and if we are the // only expression using it... // if (BinaryOperator *LHSI = dyn_cast<BinaryOperator>(LHS)) if (LHSI->getOpcode() == I->getOpcode() && LHSI->hasOneUse()) { // If the rank of our current RHS is less than the rank of the LHS's LHS, // then we reassociate the two instructions... unsigned TakeOp = 0; if (BinaryOperator *IOp = dyn_cast<BinaryOperator>(LHSI->getOperand(0))) if (IOp->getOpcode() == LHSI->getOpcode()) TakeOp = 1; // Hoist out non-tree portion if (RHSRank < getRank(LHSI->getOperand(TakeOp))) { // Convert ((a + 12) + 10) into (a + (12 + 10)) I->setOperand(0, LHSI->getOperand(TakeOp)); LHSI->setOperand(TakeOp, RHS); I->setOperand(1, LHSI); // Move the LHS expression forward, to ensure that it is dominated by // its operands. LHSI->getParent()->getInstList().remove(LHSI); I->getParent()->getInstList().insert(I, LHSI); ++NumChanged; DEBUG(std::cerr << "Reassociated: " << *I/* << " Result BB: " << I->getParent()*/); // Since we modified the RHS instruction, make sure that we recheck it. ReassociateExpr(LHSI); ReassociateExpr(I); return true; } } return Changed; } // NegateValue - Insert instructions before the instruction pointed to by BI, // that computes the negative version of the value specified. The negative // version of the value is returned, and BI is left pointing at the instruction // that should be processed next by the reassociation pass. // static Value *NegateValue(Value *V, BasicBlock::iterator &BI) { // We are trying to expose opportunity for reassociation. One of the things // that we want to do to achieve this is to push a negation as deep into an // expression chain as possible, to expose the add instructions. In practice, // this means that we turn this: // X = -(A+12+C+D) into X = -A + -12 + -C + -D = -12 + -A + -C + -D // so that later, a: Y = 12+X could get reassociated with the -12 to eliminate // the constants. We assume that instcombine will clean up the mess later if // we introduce tons of unnecessary negation instructions... // if (Instruction *I = dyn_cast<Instruction>(V)) if (I->getOpcode() == Instruction::Add && I->hasOneUse()) { Value *RHS = NegateValue(I->getOperand(1), BI); Value *LHS = NegateValue(I->getOperand(0), BI); // We must actually insert a new add instruction here, because the neg // instructions do not dominate the old add instruction in general. By // adding it now, we are assured that the neg instructions we just // inserted dominate the instruction we are about to insert after them. // return BinaryOperator::create(Instruction::Add, LHS, RHS, I->getName()+".neg", cast<Instruction>(RHS)->getNext()); } // Insert a 'neg' instruction that subtracts the value from zero to get the // negation. // return BI = BinaryOperator::createNeg(V, V->getName() + ".neg", BI); } bool Reassociate::ReassociateBB(BasicBlock *BB) { bool Changed = false; for (BasicBlock::iterator BI = BB->begin(); BI != BB->end(); ++BI) { DEBUG(std::cerr << "Reassociating: " << *BI); if (BI->getOpcode() == Instruction::Sub && !BinaryOperator::isNeg(BI)) { // Convert a subtract into an add and a neg instruction... so that sub // instructions can be commuted with other add instructions... // // Calculate the negative value of Operand 1 of the sub instruction... // and set it as the RHS of the add instruction we just made... // std::string Name = BI->getName(); BI->setName(""); Instruction *New = BinaryOperator::create(Instruction::Add, BI->getOperand(0), BI->getOperand(1), Name, BI); // Everyone now refers to the add instruction... BI->replaceAllUsesWith(New); // Put the new add in the place of the subtract... deleting the subtract BB->getInstList().erase(BI); BI = New; New->setOperand(1, NegateValue(New->getOperand(1), BI)); Changed = true; DEBUG(std::cerr << "Negated: " << *New /*<< " Result BB: " << BB*/); } // If this instruction is a commutative binary operator, and the ranks of // the two operands are sorted incorrectly, fix it now. // if (BI->isAssociative()) { BinaryOperator *I = cast<BinaryOperator>(BI); if (!I->use_empty()) { // Make sure that we don't have a tree-shaped computation. If we do, // linearize it. Convert (A+B)+(C+D) into ((A+B)+C)+D // Instruction *LHSI = dyn_cast<Instruction>(I->getOperand(0)); Instruction *RHSI = dyn_cast<Instruction>(I->getOperand(1)); if (LHSI && (int)LHSI->getOpcode() == I->getOpcode() && RHSI && (int)RHSI->getOpcode() == I->getOpcode() && RHSI->hasOneUse()) { // Insert a new temporary instruction... (A+B)+C BinaryOperator *Tmp = BinaryOperator::create(I->getOpcode(), LHSI, RHSI->getOperand(0), RHSI->getName()+".ra", BI); BI = Tmp; I->setOperand(0, Tmp); I->setOperand(1, RHSI->getOperand(1)); // Process the temporary instruction for reassociation now. I = Tmp; ++NumLinear; Changed = true; DEBUG(std::cerr << "Linearized: " << *I/* << " Result BB: " << BB*/); } // Make sure that this expression is correctly reassociated with respect // to it's used values... // Changed |= ReassociateExpr(I); } } } return Changed; } bool Reassociate::runOnFunction(Function &F) { // Recalculate the rank map for F BuildRankMap(F); bool Changed = false; for (Function::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) Changed |= ReassociateBB(FI); // We are done with the rank map... RankMap.clear(); ValueRankMap.clear(); return Changed; }